Adventure games are my favorite genre so I can't possibly pick one but here are some of my favorites:
LucasArts
Monkey Island I-IV (with IV being slightly below the rest)
Day of the Tentacle
Sam & Max hit the road
Indiana Jones and the fate of Atlantis
Full Throttle (short but good)
Grim Fandango
Sierra
Space Quest I-V (didn't care much about VI even if it was the first SQ I played)
King's Quest VI (the only KQ I own

ops: )
Quest for Glory I-V (excellent mix of adventure and RPG, probably my favorite 'Quest' series)
Gabriel Knight I-III
Revolution
Broken Sword I-III
Beneath a Steel Sky
Access
The Tex Murphy games (too bad I haven't played Mean Streets or Martian Memorandum yet)
I'd also mention the heavily underrated game Toonstruck (Burst, 1996) and the old Zork games from Infocom (as well as the modern Zork Grand Inquisitor)